Active implanted medical devices comprise particularly cardiac pacemakers, defibrillators, neurological devices, pumps for the delivery of medical substance (so-called diffusion pumps) and cochlear implants. These devices, once put in place, are programmed from the exterior by use of a remote or distant console called a "programmer". The invention, however, is not limited to active implanted medical devices. Indeed, although the invention is described with reference to active implanted medical devices, the invention also applies to medical devices that are not implanted (for example, devices carried by the patient), and to non-active medical devices (for example, devices, whether or not implanted, that are without a source of energy (e.g., a battery) and which use for the emission of signals to the exterior a portion of the energy of an interrogating signal that is applied to the device), and to autonomous devices other than medical devices. The reference to an "implant" in the description is, therefore, not in itself restrictive to implanted medical devices and should be construed synonymous with autonomous device (except when used as an adverb to a medical device).
The verification of parameters of the implant or the transmission of information to be recorded (stored) by the implant is realized by electromagnetic inductive coupling, called "telemetry" in the technique in question. Thus, a programmer communicates with an autonomous device by telemetry.
Each programmer is provided with a receiver head that is placed in face of, that is in the proximity to the site of, the implant. The head comprises a coil (also called an antenna) that collects the magnetic field generated from or by the implanted device. The programmer also is configured to send information to the implant by electromagnetic means. This is typically done by causing a current in the programmer coil to oscillate, which current will provoke a voltage at the output of the reception coil of the implant, and produce a voltage on the implant coil leads. These voltages are collected (sensed) and decoded by the implant.
Signals emitted by the implant are collected by the programmer coil, amplified, filtered, digitized and decoded by the programmer circuits. This allows one to realize a transmission in two directions between the implant and the programmer (i.e., bi-directional telemetry). It is noted that for each of the programmer and the autonomous device, the coil used for receiving signals can also be used to transmit signals, although different coils or different combinations of coils may be used for receiving and sending.
It has been previously proposed to operate data transmission in a synchronous mode between an implant and a programmer. In the known technique, the programmer possesses a clock that defines the rhythm (rate) of data transmission, which rate is imposed on the implant when the implant has data to emit in the direction of the programmer. This technique, if it is able to benefit from the advantages of a synchronous transmission, is nevertheless limited in its possibilities of implementation because it presupposes that the implant is able to measure, establish, and maintain the cadence of the transmission rate imposed by the programmer.
In practice, it is indispensable to limit the transmission rate if this condition is to be satisfied in a quasi-certain manner, independent of the type of implant that is interrogated by the programmer. In others words, it is desirable, if not necessary, to find a compromise between the speed of the synchronous transmission and the security of transmission. Too high a speed will present a risk of losing the synchronism during the transmission, and, therefore, result in the erroneous interpretation by the programmer of the transmitted data.